This invention relates to oxide superconductor assemblies in which oxide superconducting elements are electrically decoupled from one another. The invention further relates to oxide superconductor conductors having high transverse matrix resistivity and methods of their making.
Many applications of high temperature oxide superconductors involve conductor performance in time-varying magnetic fields with very stringent AC loss requirements. Mitigation of AC losses in superconducting oxides involves control over filament dimension, conductor matrix dimensions, matrix resistivities perpendicular to the magnetic field (transverse resistivity) and the critical current.
AC losses may be attributed to three different phenomena: hysteresis loss, eddy current loss, and coupling loss. Hysteresis loss describes the effect of a magnetic field on a superconducting filament due to hysteresis magnetization. Eddy current losses represent current loops in the matrix that create a magnetic field which opposes a change in the applied field. Coupling losses are similar to eddy current losses where a significant portion of the current loop may be loss-less inside two or more superconducting filaments in a multifilament composite.
Hysteresis loss is generated only in the superconductor filament and is predominantly proportional to filament dimension. Therefore, to mitigate hysteresis loss, it is desirable to reduce the filament diameter. Filament diameters on the order of 140 xcexcm or less are considered minimally effective in the mitigation of hysteresis loss in oxide superconducting composites.
Eddy currents are generated only in the matrix and coupling currents involve current loops between two or more filaments connected through the matrix in a multifilamentary composite. Both eddy current losses and coupling losses are inversely proportional to the matrix resistivity. For coupling currents, the relevant resistivity is transverse to the filament axis. In addition, coupling losses depend upon the superconductor filament critical current density and the twist pitch of the filaments in the multifilamentary composite. Thus, AC losses due to coupling and eddy currents may be mitigated by decreasing the twist pitch of the superconducting filaments and increasing the resistivity (particularly the transverse resistivity) of the matrix.
Losses may be effectively mitigated only for twist pitches that are short relative to the diameter of the conductor composite. The twist pitch is defined as the longitudinal distance over which a filament traverses in a complete revolution around the conductor back to its angular starting point. As the twist pitch approaches the wire diameter, the angle of the filaments increases rapidly, as does the torsional strain, and the dependency of loss on twist pitch weakens.
The effective transverse resistivity is complicated by the unusual oxide superconductor grain morphology. Typically, both the overall composite and the superconducting filaments within the composite are aspected, with a cross sectional width, w, to thickness, d, ratio (w/d) on the order of 10 or more. Power losses are inversely proportional to the filament thickness (which scales as 1d) or width depending upon field orientation. To achieve the same loss level in a composite with an aspect ratio of 10, the matrix resistivity must be at least 100 times greater than in a non-aspected conductor.
Thus it is desirable to use fine dimension, twisted filaments in oxide superconducting composites having a high matrix resistivity. Because twisting becomes ineffective for twist pitches that approach the conductor""s cross sectional dimensions and introduce filament strain, the most effective means of mitigating AC losses is to increase transverse resistivity of the matrix.
Attempts to prepare oxide superconducting composites with high matrix resistivities have been reported in the prior art. Many of the reported composites use a matrix alloyed with an element selected to reduce the overall conductivity of the matrix, e.g., Agxe2x80x94X, where X is Au, Al, etc. Shiga et al. in U.S. Pat. No. 5,296,456 describe an oxide superconducting composite in which a ceramic superconductor is sheathed in a noble metal. The noble metal is alloyed with metals such as Zn, In, Cd, Cu, Mg, Be, Ni, Fe, Co, Cr, Ti, Mn, Zr, Al, Ga and rare earth elements to form a low conductivity layer. However, such alloys are not effective in reducing the conductivity of the matrix to levels considered effective for the mitigation of power losses. Further, both the longitudinal and transverse resistivity of the matrix is reduced. This is disadvantageous because it inhibits a high conductivity electrical shunt or current bypass should the superconducting pathway fail.
Sumitomo Electric Co. in EP 638,942 describe a twisted, multifilamentary oxide superconducting composite, in which each individual filament strand is surrounded by a 10 wt % Au/Ag alloy layer having a higher resistivity than the silver matrix. Such a composite suffers from several disadvantages. First, the Auxe2x80x94Ag alloy is of insufficient resistivity to mitigate AC losses in magnetic fields of 0.1-0.2 T, which is of interest in many applications. Secondly, the high resistance layer is directly surrounding the superconducting filament, precluding a more conductive silver matrix to act as a longitudinal electrical shunt in the event that a filament loses superconductivity. In addition, the resultant cable is very expensive to make.
Wagner et al. in U.S. Pat. No. 4,990,491 discloses a multifilamentary low temperature superconducting (LTS) wire (Nb3Sn) with an outer NiO coating. Copper or bronze clad filaments are plated with a metallic nickel layer, which is then converted to NiO in the heat treatment used to form the Nb3Sn. The architecture of the cable permits insulation of one multifilamentary strand from another, but does not decouple each superconductor filament as is required to mitigate coupling losses. Further, the AC losses in oxide superconductor composites are quite different than in low temperature superconductor (LTS) composites. The oxide superconductor filaments are usually larger and the matrix resistivity is smaller than in LTS composites. Weak links and the anisotropy of the high Tc superconductor grains produce a critical current density (Jc) that varies with the local filament chemistry and grain orientation. Thus, composite geometries useful in the LTS field are not readily applicable to the oxide superconductor composites.
Other references report the use of insulating layers or sheets in the construction of a multifilamentary oxide superconductor composite. EP 503,525 describes a multifilamentary composite in which an intermediate layer made up of a high resistance metal, such as CuNi, is placed between multifilamentary tapes making up the composite. Only low level loss reduction is achieved. While this reduced coupling between multifilamentary tapes, it does not satisfactorily reduce losses within each tape.
In a similar fashion, EP 650,205 describes multifilamentary oxide superconductor composite tapes prepared from multiple tape layers spirally wound on a cable form. In order to reduce AC losses due to coupling between multifilamentary tapes, an intermediate insulating layer is wound between individual tape windings. As in EP 503,525, this architecture may reduce coupling between multifilamentary tapes, but it does not reduce losses within each tape.
Thus, the prior art attempting to increase the resistivity between individual superconducting filaments for the mitigation of AC losses has not been satisfactory. There remains a need to provide an oxide superconductor composite which possesses sufficient transverse matrix resistivity to reduce AC power losses, but which retains sufficiently low longitudinal resistivity in contact with the superconducting filament to serve as a conductive shunt. Furthermore, such a composite should be prepared under a cost-effective, manufacturing condition.
It is the object of the present invention to provide an oxide superconducting composite which possesses a high transverse matrix resistivity, but which retains sufficiently low longitudinal resistivity in contact with the superconducting filament to serve as a conductive shunt.
It is a further object of the present invention to provide a superconducting composite having the above features and having sufficient toughness and flexibility to be assembled into a cable.
It is a further object of the present invention to prepare a high resistivity layer within an oxide superconductor composite.
It is yet a further object of the invention to prepare a high resistivity layer onto each strand of an oxide superconductor composite cable so as to electrically decouple the superconducting filaments.
These and other objects of the invention will be made clear with reference to the description of the invention which follows.
In one aspect of the invention, an oxide superconducting cable, is provided having a plurality of strands comprised of at least one oxide superconductor filament sheathed in a ductile and conductive metal matrix, at least half of strands further comprising an adherent, substantially continuous high resistivity coating substantially surrounding said at least half of said strands, wherein the strands are positioned and arranged to form a cable.
In another aspect of the invention, a cable is provided having a plurality of strands, each of said strand comprised of at least one filament comprised of a precursor to an oxide superconductor sheathed in a ductile and conductive metal matrix. At least half of said strands further comprise an adherent, substantially continuous metal coating on the outer surface of said at least half of said strands, said metal capable of conversion into a high resistivity material, wherein the strands are positioned and arranged to form a cable.
In yet another aspect of the invention, a cable is provided having a plurality of strands, each of said strands comprised of at least one filament comprised of a precursor to an oxide superconductor sheathed in a silver-based matrix. At least one of said strands further comprising an adherent, substantially continuous silver-based intermetallic material coating on the outer surface of said strand, said intermetallic material capable of conversion into a high resistivity material, wherein the strands are positioned and arranged to form a cable. Another aspect of the invention includes an oxide superconducting strand at least one oxide superconductor filament sheathed in a ductile and conductive metal matrix and an adherent high resistivity coating substantially surrounding the outer surface of the sheathed filament. An oxide superconducting cable of the invention is characterized in that the AC losses at an alternating magnetic field of 0.1 T rms is less than about 10 mW/A-m. The strand has a superconducting oxide volume fraction of about 0.1 to about 0.5, and preferably of about 0.25 to about 0.4.
As used herein xe2x80x9cfilamentxe2x80x9d refers to a single, substantially continuous elongated oxide superconductor domain. Further, reference to a xe2x80x9csuperconducting filamentxe2x80x9d or an xe2x80x9coxide superconducting filamentxe2x80x9d and the like includes filaments made up of a precursor to the desired oxide superconductor. A xe2x80x9csuperconductor precursorxe2x80x9d as that term is used herein, is meant any material (e.g., metals, salts or oxides) that can be converted into the desired oxide superconductor by heat treatment under suitable conditions. xe2x80x9cMatrixxe2x80x9d refers to a material or homogeneous mixture of materials which supports or binds the superconducting oxides or their precursors disposed within or around the matrix because of their malleability and low resistivity. By xe2x80x9cconductive metal matrixxe2x80x9d as that term is used herein, it is meant a metal matrix sufficiently conductive so that it may act as a longitudinal electrical shunt. Thus, the conductive metal matrix preferably does not have a resistivity greater than about 5 xcexcxcexa9-cm. xe2x80x9cHigh resistivityxe2x80x9d, as the term is used herein, means a resistivity greater than 100 xcexcxcexa9-cm, and preferably in the range of about 1 mxcexa9-cm to 1 xcexa9-cm.
xe2x80x9cStrandxe2x80x9d refers to one or more filaments substantially surrounded or supported by a metal matrix. A xe2x80x9cmonofilamentxe2x80x9d strand refers to a strand containing a single filament. A xe2x80x9cmultifilamentxe2x80x9d strand refers to strand containing two or more filaments embedded in or supported by the same metal matrix. A xe2x80x9ccablexe2x80x9d, xe2x80x9ccabled conductorxe2x80x9d or xe2x80x9ccable compositexe2x80x9d as those terms are used herein means an assembly of strands, which may be monofilamentary or multifilamentary, created by transposing or otherwise arranging the strands in conventional cable architectures.
In preferred embodiments, each strand of the cable is substantially surrounded, and preferably diffusion bonded, by the high resistivity coating. Each strand may be diffusion bonded to a neighboring strand. The strand may be a monofilament, a multifilament or a preassembled cable.
In other preferred embodiments, the high resistivity coating of the cable comprises a metal oxide, preferably selected from the group consisting of oxides of tin, bismuth, gallium, antimony, zinc, iron, nickel, niobium, tantalum, zirconium and indium and alloys thereof with each other and silver. The coating thickness is in the range of about 1 xcexcm to about 5 xcexcm and preferably in the range of about 2 xcexcm to about 3 xcexcm. In preferred embodiments, the high resistivity coating has a resistance greater than about 10 xcexcxcexa9-cm, and preferably greater than about 1 mxcexa9-cm, but less than about 10 xcexa9-cm.
In other preferred embodiments, the filament diameter is less than 250 xcexcm, preferably less than 140 xcexcm and even more preferably less than 100 xcexcm. In other preferred embodiments, the cable strands are positioned and arranged so as to form a cable selected from the group consisting of concentric, bunched and rope lay cables and higher order cables form therefrom.
In other preferred embodiments, the high resistivity layer comprises two substantially continuous layers having a conductive metal layer disposed therebetween. Non-continuous domains of the high resistivity material may be dispersed within the conductive metal matrix in an amount insufficient to significantly increase the resistivity of the conductive metal matrix. In other embodiments, the cable has an aspect ratio of greater than one.
In yet another aspect of the invention a method is provided for the manufacture of a superconducting article having a high transverse matrix resistivity. A ductile predecessor coating is applied to a plurality of strands, each said strand comprised of at least one oxide superconductor filament or a precursor thereto sheathed in a ductile and conductive metal matrix, wherein the ductile predecessor is capable of conversion into a high resistivity material, assembling the plurality of strands into a cable and converting the ductile predecessor into a high resistivity material, wherein the first and second steps can be performed in any order. A method is provided for the manufacture of a superconducting article comprising fine filaments and a high resistivity layer which maintains filament integrity and the integrity of the high resistivity layer throughout the manufacturing process.
In yet another aspect of the invention, an oxide superconducting cable having a high transverse matrix resistivity is prepared by cabling a plurality of strands, each said strand comprised of at least one oxide superconductor filament or a precursor thereto sheathed in a conductive metal matrix, contacting the cable with a ductile predecessor so as to form a ductile predecessor layer on the cable strands, wherein the ductile predecessor is capable of conversion into a high resistivity material, and converting the ductile predecessor into a high resistivity material.
In preferred embodiments, the ductile predecessor is converted into a high resistivity material by oxidizing the metal into the corresponding metal oxide. In other preferred embodiments, the ductile predecessor layer is applied by contacting the filaments with the ductile predecessor in a molten or liquid form, electroplating, ion implantation, physical vapor deposition, electroless deposition.
In preferred embodiments, the method further comprises heating the coated cable after assembly of the cable to adhere the strands to each other. The heat treatment may occur at a temperature at about or above the melting point Tm of the ductile predecessor.
In preferred embodiments, the conversion step is carried out at superatmospheric oxygen pressures and at a temperature sufficient to convert to the ductile predecessor into a high resistivity material, wherein preferably the oxygen pressure is in the range of 15-3000 psi, and wherein preferably the total pressure is in the range of about 15-60,000 psi. The temperature is in the range of about 400xc2x0 C. to about 700xc2x0 C. In other preferred embodiments, the conversion step is carried out under conditions additionally sufficient to convert the oxide superconductor precursor to the oxide superconductor.
In another preferred embodiment, the cable is further heated under temperatures and pressures sufficient to convert the oxide superconducting precursor to the oxide superconductor. In another preferred embodiment, the cable is further subjected to deformation processing sufficient to texture the oxide superconductor or precursor thereto, said deformation processing occurring prior to the conversion of the ductile predecessor into a high resistivity material. The deformation includes rolling, pressing, turks heading, drawing, extruding and twisting. In other preferred embodiments, the strand is twisted prior to the cabling step. In other preferred embodiments, the high resistivity coated cable is subjected to a heat treatment selected to heal microcracks in the oxide superconductor.